Plane (geometry)

The question goes like this-
Find the equation of the plane containing the lines through the origin with direction cosines proportional to 2,1,-2 and 5,2,-3.
I have done most of the part,but got confused on this part-
I got two equations
5a+2b-3c=0
2a+b-2c=0
I just don't know how to apply cross multiplication for a,b and c.